bout 150 years ago, when liberalism was revolutionary, Pope Pius IX condemned it in his ''Syllabus of Errors'' with such vigor that he was smilingly referred to by non-Italians as ''Pio No-No.'' At the time, everyone knew what liberalism was: a political faith in the pre-eminent value of private property, a free market, a free press, freedom of religion and the reassignment of privilege from birth to wealth.

William F. Buckley Jr., one of the most prominent of American public intellectuals, has been one of those most responsible for the 20th-century redefinition of liberalism in America as ''conservatism.'' It has been greatly due to his work as a writer, editor and occasional financial angel that the word ''conservative'' no longer means a noblesse-oblige Tory or a God-and-King authoritarian, even in Latin America, and that the word ''liberal'' has been turned into a term of opprobrium embracing socialism (and every other ism) on the American left. Buckley's success at pushing the American public philosophy in his direction has been genuinely momentous. Many can still remember when New York progressives reckoned him as just another unreconstructed Coolidgeite, with Paleolithic suspicions of state-socialist innovations like the progressive income tax and possibly the Postal Service. Now Buckley can (and often does) take credit for the Goldwaterization of the Republican Party, a process that has driven thousands of yellow-dog Yankee Republicans like me blinking into the Platonic light of the political wilderness.

But in this book (and, as he argues, more importantly) Buckley is a Christian. It is not proverbial in America to ask, ''If you're so smart, why do you still believe in God?'' Not yet. Some Americans may hanker to ask the rather different question, ''If you're such a believer, why are you still rich?'' which stems from a low-church, largely Protestant-evangelical strain of American thought that prefers citizens to be vaguely ashamed of wealth or brains. No way, however, is Buckley going to be abashed about such family legacies as his intellect and fortune -- or his religion. ''Nearer, My God'' is a book that celebrates and propounds Buckley's Christian faith and the Roman Catholic Church to which he has belonged all his life.

This is not something Buckley does often. Indeed, as he reminds his readers, he does it quite rarely. ''I am not trained in the devotional mode, nor disposed to it.'' Of Buckley's nearly 40 other books, the overwhelming majority are on political themes, and except for his first, the interdenominational ''God and Man at Yale,'' ''Nearer, My God'' is the only one devoted to religious doctrine and its consequences. It is ''an autobiography of faith,'' a personal topic, and Buckley considers the political to be separate from the personal. Such criteria turn ''Nearer, My God'' at the outset from a treatise to a personal essay, perhaps the most personal of all of Buckley's books. The essay form is itself a legacy of the American public square and its evangelical suspicion of any extended arguments from first principles. Nevertheless the book is also a doctrinal apologia for Roman Catholicism. In fact, one of its several pleasures is in how well apologetic theology, the impersonal Catholic theological form perfected in the late 17th century, is subtly allowed to shape this, perhaps the least formal of all forms in prose. Magisterial Catholic doctrine makes the melding of apologia and autobiography difficult. Pascal's attempt was never completed. (Its fragments are the incomparable ''Pensees.'') By his own account, Buckley has read some apologias, and it shows in the order of his topics.

America's most prominent religious spokespeople these days are the likes of Billy Graham, about whose own recently published spiritual autobiography a reviewer said in these pages, ''Billy Graham's gift has been to appreciate that in matters of faith there is no approach too simple, no argument too crude, no question too basic.'' But Graham's Christianity is the variety of Protestantism with the greatest suspicion of intellect. Buckley's Catholicism is unafraid of book learning or dialectical brilliance, and its questions can challenge the sharpest and most cultivated minds. Though Buckley quotes large numbers of Protestants in this book, they are mostly Protestants who ''poped'' (converted to Catholicism), like Cardinal Newman, Ronald Knox, Richard John Neuhaus and Arnold Lunn, and whose ''poping'' stemmed more from thoughtful consideration than any sudden access of irresistible grace. The few unconverted Protestants who seem to play a part are Bishop Butler, Reinhold Niebuhr, Martin Luther King Jr., Charles Colson, Charlton Heston and Buckley's wife, Pat. Repeating the medieval saw that ''nothing contrary to reason'' is required by true religion, Buckley uses a panel of the ''poped'' to examine in their own words questions Buckley thinks important. These range from the oldest and most fundamental (the existence of God, the unique historicity of Jesus) to the most current and pragmatic (divorce, priestly celibacy, the ordination of women). These current questions do not include abortion, to which Buckley devotes a quick footnote; but they do include contraception (Connecticut's Buckley is not entirely prepared to reverse Griswold v. Connecticut) and Vatican II, whose reforms move Buckley to irritated irony, aristocratic objections to unison prayer and a general attitude that is a whole lot less submissively ultramontane than the one he preaches in the rest of the book.

But the book never stops being an autobiography. It begins in Buckley's secondary schools -- an English Catholic prep school, where Buckley found himself relating to the Virgin Mary by envisioning his own mother, and a New England Protestant prep school where a teacher allowed Bach's ''St. Matthew Passion'' to take over his curriculum. It goes on to his service in World War II and in the C.I.A., his beginnings as a writer, his marriage to Pat and the upbringing of their son, his writing winters in Gstaad, his service as lector in his local parish in the 1960's, the secularization of the convent next to his house on the Connecticut shore, meeting Pope John Paul II and standing godfather. As in so many of his other books, the leading members of his old conservative gang put in an appearance, one after another, from siblings, in-laws and old friends to the newest recruits to National Review. Then, like the best of essays, Buckley's ends where it began, with the question of religious practice in Christian schools, and with the figure of his own mother, his earliest model of the Madonna, who died in 1985 and to whom the book is dedicated.

So here is an American political intellectual writing about religion from the inside. Not novel, but risky. Cohabitation with politics and philosophy has vexed religion in dreadful ways in other countries. In the United States, as Buckley well knows, ''traditional culture is Protestant'' and its Protestantism is of the evangelical (post-Great Awakening) variety. In other words, our central religious tradition is anti-Catholic. The history of Catholicism in America is a three-century trial of misunderstanding, prejudice and persecution, aimed at every variety of Catholic from Spanish to Irish. But the history of American Catholicism is also a history of social advancement. In the United States, in sharp contrast to Europe and Latin America, Catholicism has been a religion of the masses versus the classes, and it has risen according to the American dream. Buckley is most prominent among those who have lent it the polish and aristocratic promise of English Catholicism -- a promise about which the descendants of the peasant Irish, who brought the faith here to American cities, seem never to complain. After all that Catholics in America have been through, perhaps they may be forgiven for sitting in suburban pews and celebrating what Walker Percy once called ''Property Rights Sunday.''

Buckley's many loyal readers will be pleased with this book. One wishes it could remedy the growing American ignorance of Christianity with its brief, rather elegant introduction to Catholic doctrine, but I do not know that it will change many minds. No matter. Even if it does not explain the moral and political stands of Catholics like Bartolome de Las Casas, Dorothy Day, Daniel Berrigan, Percy, Pascal or even Pius IX, it is certain to provide many insights into the mind of a polished writer, passionate logophile and dean of controversy, and it will do much to explain how the hopefully changeless faith of a Catholic jibes with the politics of a late 20th-century American political conservative. (Or is it liberal?)

William R. Everdell teaches English at St. Ann's School and is the author of ''The First Moderns.''